1. Field of the Invention
The invention relates to a constant-voltage generator circuit and semiconductor memory, and more particularly, to the improvements of a circuit for generating a fine-adjusted constant voltage and low-voltage-driven memory.
2. Description of the Related Art
Recently, pattern microminiaturization is making a rapid progress in large scale integrated circuit (hereinafter referred to as "LSI") devices because of the needs for the smaller size and higher capacity, and the higher speed of transistor operation. This, in turn, reduces the resisting voltage of MOS transistors. As for semiconductor memory for storing information, for example, designers strive to reduce its driving voltage.
In order to supply high precision constant voltages to the low-voltage-driven semiconductor memory, designers usually use constant voltage generator circuits using a band gap or constant voltage generator circuits using the threshold voltage of field effect transistors.
However, adjusting steps of the constant voltage are limited to the selection ranges which depend on the thresholds of diode-configured transistors of a constant voltage circuit. Accordingly, the voltage adjustment fineness may become "rough" and a fine adjustment of the power supply voltage may need to be done for each load circuit because the thresholds vary with unevenness in transistor manufacturing.
Here, related arts of the invention will be explained. For example, as shown in FIG. 1 a first constant voltage generator circuit supplying a constant voltage V.sub.DD to SRAM (static random access memory) comprises a transistor selection circuit 1, a load resistor RL1, and four transistors T11-T41. Note that the circuit is arranged in a manner where the back-gates BG1-BG4 of the four transistor T11-T41 are biased with a common voltage.
Specifically, the four transistors T11-T41 are composed of n-type field effect transistors, each of which is diode-configured. The four transistors T11-T41 are serially connected with the drain of transistor T11 connected to one end of a load resistor RL1 and connected to an output OUT. The other end of the load resistor RL1 is connected to the power source line V.sub.CC and the source of the transistor T41 is connected to the ground line V.sub.SS. The back-gates BG1-BG4 of the four transistors T11-T41 are connected together to the ground line V.sub.SS. And, each of source-drain connection points of the transistors T11-T41 is connected to a transistor selection circuit 1.
The transistor selection circuit 1 comprises three switching devices TS1-TS3, the connections of which are controlled on the basis of external control signal S. The switching device TS1 is connected between the source-drain connection points of the transistors T11 and T21 and the transistors T21 and T31. The switching device TS2 is connected between the source-drain connection points of the transistors T21 and T31 and the transistors T31 and T41. The switching device TS3 is connected between the source-drain connection point of the transistors T31 and T41 and the ground line V.sub.SS.
The above-described constant voltage generator circuit functions as follows: when switching devices TS1-TS3 are selected and connected on the basis of an external control signal S, diode-configured transistors T21-T41 are selected; this causes the voltage between the power source line Vcc and the ground line V.sub.SS to be divided by the load resistor RL1 and the serially connected transistors T11-T41, where each resistance of the transistors T11-T41 in the conducting (or ON) state exhibits a value which depends on the substantially constant threshold VTH; and a constant voltage VDD is generated at the output OUT.
Further, a second constant voltage generator circuit according to related arts of the present invention comprises a transistor selection circuit 2, a load resistor RL2, and four transistors T12-T42 as shown in FIG. 2. In this circuit, the back-gates of the four transistors T12-T42 are biased with different voltages.
As in the first constant voltage generator circuit, the four transistors T12-T42, each of which is in a diode configuration, are connected in a series. The drain of transistor T12 is connected to one end of the load resistor RL2 and the output OUT. The other end of the load resistor RL2 is connected to the power source line V.sub.CC and the source of the transistor T42 is connected to the ground line VSS.
The back-gate BG1 of the transistor T12 is connected to the source-drain connection point of the transistors T12 and T22, the back-gate BG2 of the transistor T22 is connected to the source-drain connection point of the transistors T22 and T32, the back-gate BG3 of the transistor T32 is connected to the source-drain connection point of the transistors T32 and T42. And the back-gate BG4 of the transistor T42 is connected to the ground line V.sub.SS.
The transistor selection circuit 2 comprises three switching devices TS1-TS3 and their connections are controlled on the basis of an external control signal S. The switching device TS1 is connected between the source-drain connection points of the transistors T12 and T22 and the transistors T22 and T32. The switching device TS2 is connected between the source-drain connection points of the transistors T22 and T32 and the transistors T32 and T42. The switching device TS3 is connected between the source-drain connection point of the transistors T32 and T42 and the ground line V.sub.SS.
The constant voltage generator circuit functions as follows: when switching devices TS1-TS3 are selected and connected on the basis of an external control signal S, diode-configured transistors T21-T41 are selected; this causes the voltage between the power source line V.sub.CC and the ground line V.sub.SS to be divided by the load resistor RL2 and the transistors T12-T42, where each resistance of the transistors T11-T41 in the conducting state exhibits a value which depends on a different threshold VTH; and a constant voltage VDD is generated at the output OUT.